1. Field of the Invention
The invention relates to a wood-based lignocellulosic fibrous material.
2. Discussion of Background Information
Lignocellulosic fibers are used, inter alia, for the production of paper and paperboard. A large number of industrially produced lignocellulosic fibers are known, their properties differing greatly:
Groundwood designates fibers which are produced by mechanical defibering of the fiber composite by beating or grinding units. During the production of groundwood, barely any woody substance is broken down. The biomass originally used is found almost completely again in the groundwood. The production of groundwood requires a high use of energy. Newer processes for the production of groundwood attempt to improve the fiber characteristics and/or to reduce the energy demand by pre-treating the wood with steam and/or chemicals. These processes include, in particular, CTMP (chemo-pulp) and TMP (thermomechanical pulp). In the case of CTMP, in the industrial application, between 1 and 5% by weight of the chemicals, based on oven-dry wood, are normally used in order to permit partial dissolution of the fiber composite. Groundwood is generally characterized by low strength properties, in particular low tearing (breaking) length, and high opacity and light scattering with a low whiteness with a high tendency to yellowing.
Chemical pulp designates fibers which are produced by chemical dissolution of the fiber composite. During the production of chemical pulp, chemicals are used which normally act on the biomass under high pressure and high temperature. With more or less comprehensive removal of the lignin and part of the carbohydrates, that is to say with a significant loss in yield, fibers are produced which have good strength properties, in particular a high tearing length, and have a good ability to be bleached to a high whiteness and with a low tendency to yellowing. The energy required for the production of the chemical pulp is obtained from the waste liquor from the pulping.
The lignin content is often not critical for the use of the fibers. As a rule, the strength level is critical, since it often limits the areas of use. Numerous processes have therefore been developed which attempt to achieve a higher strength level, even for fibers with a higher lignin content, on the basis of processes for chemical pulp production.
Such a process, which has become established in practice in individual cases, is the NSSC process. By using extremely small quantities of sulfite, in the industrial application with neutral to slightly alkaline pH values, an attempt is made to achieve the highest possible strength of the fibers with the minimum breakdown of lignin. The quantities of chemicals are in practice kept as low as possible, since the process is operated without chemical recovery and, on account of the chemicals and the organic load which arises as a result of breakdown of the lignocellulosic material, produces a high effluent loading. Fibrous materials produced in accordance with the NSSC process are normally used unbleached.
Another process is the bisulfite process, which is operated at pH values around 4. Other processes, such as the kraft process (also called the sulfate process) or the soda process, which were developed and are used intrinsically for the production of chemical pulps with minimal lignin content, have also been checked for their suitability for the production of high-yield fibrous materials.
When checking suitability for such fibrous materials, the starting point is always practical experience that, on account of the high lignin content, the fiber in the unbeaten or in the little beaten state has only unsatisfactorily low strengths and the ability to be used economically is not provided. A good overview of high-yield fibrous materials is provided by “Choosing the best brightening process”, N. Liebergott and J. Joachimedes, Pulp & Paper Canada, Vol. 80, No. 12, December 1979, T391-T395. There, for unbleached fibrous materials which were produced by various processes, the achievable strength level is given as a function of the yield and of the lignin content. As the lower limit of fibers which can be used for papermaking, the strength level is measured at 500 ml CSF (26° SR), and a comparative measurement is carried out for 300 ml CSF (41° SR) At yields of about 80%, breaking lengths of about 9-10 km at 500 ml CSF (26° SR) are achieved for spruce. The strength values increase with further beating. These already comparatively high values are achieved by pulping in the acid p13 range (bisulfite pulping, acid sulfite pulping). For fibers from neutral and alkaline pulping (neutral sulfite pulping, kraft and soda pulping), considerably lower strength values are indicated, which additionally have to be produced by a use of defibering and beating energy which is many times higher. This can be read from the higher numbers of revolutions of the PET beating unit which are needed to achieve a freeness of 500 ml CSF (26° SR) and 300 ml CSF (41° SR).